1. Field of the Invention
The present invention concerns a shim board system of the type having a shim board with an insulated substrate and a conductor trace on the substrate for generation of a magnetic field for shimming another magnetic field.
2. Description of the Prior Art
Magnetic resonance (MR) technology is a known technology for acquisition of slice images of an examination subject using nuclear magnetic resonance signals. In a magnetic resonance apparatus rapidly-switched gradient fields that are generated by a gradient system are superimposed on a static basic magnetic field that is generated by a basic field magnet. The magnetic resonance apparatus also has a radio-frequency system that radiates radio-frequency signals into the examination subject to elicit magnetic resonance signals and acquires the elicited magnetic resonance signals. In the image acquisition, having a high homogeneity of the basic magnetic field is a decisive factor for the quality of the MR images. Inhomogeneities of the basic magnetic field within an imaging volume of the magnetic resonance apparatus cause geometric distortions of the MR image that are proportional to the inhomogeneities. The field homogeneity is particularly important in imaging sequences known as fast pulse sequences, for example in an echo planar method.
Shim systems are used to improve the basic magnetic field homogeneity within the imaging volume. Shim systems can be categorized as passive shim systems and active shim systems. In a passive shim system, a number of iron plates are mounted in the examination chamber of the apparatus in a suitable arrangement.
In an active shim system, to homogenize the basic magnetic field shim coils that can be charged with direct currents are used that are, for example, fashioned as shim boards (circuit boards having a substrate with one or more conductor runs or traces thereon). Mains adaptors that deliver very constant and reproducibly adjustable direct currents are necessary for the operation of the shim coils. Active shim systems are used for, among other things, fine correction to achieve a very high homogeneity, for example in order to correct field distortions caused by the magnetic susceptibility of the examination subject at least partially positioned in the imaging volume. Among other things, shim coils for various field corrections are sealed (potted) with one another, meaning that a sandwich structure is formed that is composed of a substrate plate, a shim coil, another substrate plate, another shim coil etc. in a sequence. Such a design also can be sealed, for example, with a gradient coil unit. In order to allow a transverse passage of sealing material between the shim boards, holes in the substrate plates are arranged at points at which no shim coil conductor is located. Due to the various conductor structures, a sufficient transverse passage of the casting resin is not always to be ensured, such that a complete saturation of the interstices is not assured. Voids thereby created result in a poor resistance arcing and represent mechanical weak points that can lead to a de-lamination of the shim boards and consequently to an increased vibration level.